Selection and Design of Channel Rehabilitation Methods
watersheds continue to stabilize the sediment concentrations will decrease, requiring that the sediment
concentration of the CEM 4 and CEM 5 reaches be reviewed and, perhaps updated.
A rational approach to selecting the proper sediment supply would be approached in the following
steps:
1. Assess sediment sources such as gullies, bank erosion, overbank watershed sources, and others
to estimate the total watershed sediment yield on an annual basis using comparative surveyed
cross sections, aerial photography, watershed models, etc.; and
2. From that assessment, estimate the sediment sources that could be eliminated using drop pipes,
bank stabilization, grade control, and land use management practices to determine a best-
practice sediment supply for the watershed.
The sediment transport capacity of the channel reach would then be computed using the following
steps:
3. Develop a sediment rating curve similar to Figure 6.16 either from measured data if available
or from sediment transport computations;
4. Generate a flow-duration curve, i.e., a relationship between the discharge and the percentage
of time during the year that a particular discharge occurs;
5. Compute the annual sediment yield as the summation of products of the rating curve and the
flow-duration curve; and
6. Adjust either the sediment rating curve using grade control, or the flow-duration curve using
reservoir detention to meet the best-practice sediment supply for the watershed.
Standard computational procedures could then be used to check steady discharge or long-term
simulation of the channel response. This proposed procedure is more intensive; however, additional
planning elements and solution methods could be considered, and detail analysis is generally only a small
fraction of construction costs for watershed stabilization.
Comparison of the annual yield from the DEC survey reaches (Table 6.1) indicates that sediment
yield has decreased 20%, as expressed as an average of all reaches. Segregating those reaches with and
without grade control structures indicates that those reaches with grade control structures have decreased
sediment yield by 36%, while reaches without grade control have increased sediment yield by an average
of 8%, compared to 1993 levels. Long-term records are required before definitive conclusions can be
made regarding effectiveness of these structures.
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